Medline Plus
Reference to diseases, nutrition, symptoms, tests and more from the National Library of Medicine.

PDRHealth
From the creators of the Physicians' Desk Reference, this site lists dosage, precautions, and potential side effects for prescription and over-the-counter drugs as well as herbal supplements.

UpToDate
Information, geared towards patients, about common medical conditions and treatments. The clinician version of this site--available through the 'home' link at the top of the page--provides the latest study results from all areas of medicine.